So basically this is how I understand these two brands for internal HDD's:
-Western Digital offers better flexibility for the customers because they have four series of drives (red, green, blue and black) which you can choose from after you're prefference.
-Seagate, for the same price offers the same amount of storage space as the green from WD at speeds of the blue, even the black series from WD.
WD is known for it's reliability but Seagate is a company from Germany so it's quality is outstanding as well.